1. Field of the Invention
This invention relates to the thermal treatment of nickel base superalloys in preparation for directional recrystallization. This invention also relates to the resultant microstructure which is amenable to directional recrystallization.
2. Description of the Prior Art
The mechanical properties of metals are strongly affected by grain boundaries. At low temperatures, grain boundaries are generally stronger than the material within the grains, but at high temperatures the reverse is true. At elevated temperatures, creep is usually observed to occur much more rapidly in fine grain materials than coarse grain materials. For this reason, coarse grained materials are usually preferred for stressed applications at elevated temperatures.
Improvements in creep properties of coarse grained materials may be obtained if the grains can be significantly elongated in the direction of stress. This elongated grain material has significantly fewer grain boundaries transverse to the stress axis; and accordingly, has improved high temperature properties in the elongation direction. As used herein, the term "elongated grain" is intended also to encompass single crystal material, which are those characterized by the absence of internal grain boundaries.
Two general techniques exist for producing such material. One method, known as directional solidification (D.S.), involves controlling the heat flow and other conditions during the solidification process to produce an elongated microstructure. This technique is discussed in U.S. Pat. No. 3,260,505 issued to VerSnyder of common assignee with the present application. The process described in this prior patent produces elongated grains in a cast structure.
The second method involves controlled recrystallization (usually after deformation). In its best known form, this type of process involves straining the material a small, but critical amount, to produce a particular dislocation density, and then heating to above the recrystallization temperature under conditions which encourage grain growth rather than grain nucleation. This process produces elongated grains in a wrought microstructure. The heating is usually performed in a moving thermal gradient and the recrystallized grains tend to grow along the axis of gradient motion. This process is reviewed in the book "The Art and Science of Growing Crystals," edited by J. J. Gilman, John Wiley Publishing Co., (1964) pages 415 through 479.
More recent references to this type of process include U.S. Pat. Nos. 3,850,702; 3,746,581 and 3,772,090.
U.S. Pat. No. 3,850,702 describes a process applicable to the gamma/gamma prime alloys in which the alloy is heat treated to produce an all gamma structure prior to straining. The straining step is performed at relatively low temperatures and reprecipitation of the gamma phase occurs during annealing. Both U.S. Pat. Nos. 3,746,581 and 3,772,090 are processes applicable chiefly to dispersion strengthened alloys. In U.S. Pat. No. 3,772,090, the strain is imparted at low temperatures, while in U.S. Pat. No. 3,746,581 the strain is imparted by hot extrusion under controlled conditions.
There exists another type of recrystallization process which is known as secondary recrystallization. In this form of recrystallization, certain existing grains grow at the expense of others without the nucleation of new grains. It is known that soluble second phase particles can effect secondary recrystallization. This is described in Trans. AIME (1939), Vol. 180, pages 163 through 192 and Trans. AIME (1961), Vol. 221, pages 439 through 445 with regard to certain iron base alloys. It does not appear that this process has been used in any practical fashion, and it further does not appear that any use has been made of thermal gradient in secondary recrystallization.
U.S. Pat. No. 3,975,219 describes a directional recrystallization process for nickel base superalloy involving hot forging under controlled conditions to produce a uniform dislocation density followed by directional recrystallization.
U.S. Pat. No. 3,677,830 describes a duplex heat treatment for nickel base superalloys including a heat treatment at a temperature between 25.degree. and 100.degree. below the gamma prime solvus followed by a second heat treatment below but within 25.degree. of the gamma prime solvus. The objective of this invention is the provision of a uniform microstructure in certain superalloys.